1. Field of the Invention
The present invention broadly relates to a data clocking system and, more particularly, is concerned with an automatic clock phase adjustment method and circuit for use in the data clocking system to insure that reliable storage of data that has been delayed by an unknown amount is achieved.
2. Description of the Prior Art
One prior art data clocking system includes a local unit and a remote unit being interconnected by clock sending and data receiving cables or the like. The local unit includes a clock pulse generator and a local data storage device, while the remote unit includes a remote data storage device. The purpose of the system is to clock out the data from the remote storage device, the data having been received at the remote storage device from some other source, and clock in that data to the local storage device and by so doing transfer the data to a local multiplexer or some other data utilization unit. Thus, in operation, clock pulses generated by the local generator are sent from the local unit on one cable to the remote data storage device for clocking out digital data from the remote device through the outer cable to the local data storage device. Concurrently, clock pulses are applied from the generator to the local storage device for clocking in or transferring the incoming data through the latter to the local data utilization unit.
In situations where digital data is being clocked from the remote unit at high rates, such as 10 to 60 MHz., clock and data propagation delays are to be expected due to variations in cable length and temperature as well as inherent characteristics of electrical circuit components. Under conditions where the sum of such delays approximately equals the closk pulse period or multiples thereof, data will not be clocked in properly at the local storage unit. This is because the clocking edge of the clock pulse being applied to the local data storage device will coincide in time with a transition in the data at the local device. This condition produces unreliable data storage or transfer at the local storage device, which ordinarily results in the loss of that particular portion of the data.
For avoiding this condition, the prior art practice has been to manually adjust or shift the phase of the clock pulses being clocked to the remote unit to insure that incoming data transitions will not coincide with the clocking edge of the clock pulses applied to the local storage device. One way that the phase of the outgoing or output clock pulses may be adjusted is by changing the length of one of the cables relative to the other such as by having several jumper cables of varying lengths so that one can be substituted for another each time a phase adjustment needs to be made. Another way is by using a one-shot multivibrator interconnected between the generator and the clock sending cable which may be manually adjusted to vary the timing of the output clock pulses. Conversely, either of these methods could be utilized to adjust the timing of the local clock pulses to the local storage device in relation to the timing of the output clock pulses.
However, these prior art methods have several disadvantages. They both require manual adjustment and continued operator attention to maintain the correct adjustment. Manual adjustment is oftentimes not feasible because it takes too much time and dramatically reduces the operating efficiency of the system. Also, because the system is often being used in environments where space limitations require extreme compactness in the installation of the equipment, such as on aircraft, it is very inconvenient and different to make the necessary manual adjustments.